Photovoltaic array systems, methods, and devices with bidirectional converter

ABSTRACT

Devices, systems and methods for operating, monitoring and diagnosing photovoltaic arrays used for solar energy collection. The system preferably includes capabilities for monitoring or diagnosing an array, under some circumstances, by using a bidirectional power converter not only to convert the DC output of the array to output power under some conditions, but also, for diagnostic operations, applying a back-converted DC voltage to the array.

CROSS-REFERENCE

Priority is claimed from U.S. Provisional Patent Applications61/418,144, filed Nov. 30, 2010, and 61/480,048, filed Apr. 28, 2011,both of which are hereby incorporated by reference.

BACKGROUND

The present application relates to photovoltaic arrays for collectingsolar energy to provide useful power, and more particularly to improveddevices, systems and methods for diagnosing and monitoring such arrays.

Note that the points discussed below may reflect the hindsight gainedfrom the disclosed inventions, and are not necessarily admitted to beprior art.

Photovoltaic systems are one of the fastest growing sources of energyand are increasingly cost-effective. The cost and reliability ofphotovoltaic systems and their components, including photovoltaicmodules and photovoltaic converters, continue to improve.

Photovoltaic modules and components are often in service for many years.An array or its components may be warranted for more than 20 years.While modules and other components are generally reliable, problems doarise. Over their service life, components are subject to malfunctionsand/or performance degradation.

Various power monitoring systems are used to determine if actualperformance meets projections. These power measurements can be: AC powerat the output of the converter, DC power at the input of the converter,DC power at the string level, and DC power at the individual module.Each of these monitoring systems provides certain useful data fordetermining if the system is operating properly. However, these powermonitoring systems are less useful for diagnosing issues whenperformance is inadequate.

Photovoltaic modules in the laboratory or manufacturing environment areoften measured to provide a current-voltage curve (“IV curve”) thatdetermines the module performance across the range of operating currentand voltage points. The IV curve also varies with the amount of solarinsulation and ambient temperature. IV curves provide a detailed insightto the internal performance of the photovoltaic module, which cannot beobtained easily once the system is installed in the field.

More recently, IV curve trace tools have been developed to enablephotovoltaic maintenance personnel to diagnose issues in the field withinstalled photovoltaic systems. IV curve trace measurements can identifythe cause of low performing modules such as soiling or shading, but theyrequire highly-trained maintenance staff to use and interpret themeasurements.

Published US application 2010/0071744 describes a PhotovoltaicInstallation with Automatic Disconnect Device. The DC disconnect devicedescribed therein uses a single DC disconnect with a control mechanismto disconnect in case of fire or other emergency. The single DCdisconnect is located at the typical manual DC disconnect locationbetween the string combiner and the converter. However, this system doesnot appear to offer any capability for string monitoring or diagnostics.

SUMMARY

The present application discloses novel systems, methods, and devicesfor a photovoltaic array with improved monitoring and diagnosticcapabilities.

In some embodiments, an array comprises a bidirectional power convertercapable of applying a DC potential to the array or to a portion of thearray to perform monitoring and diagnostic operations. In someembodiments the bidirectional converter can bias the photovoltaic arrayto variable DC voltage of either polarity, and under any solar conditionincluding day or night. These capabilities, when combined with existingDC current measurements from conventional power monitoring, can allowmeasurements such as an I-V curve trace to be created without additionalequipment.

In some embodiments, an array comprises multiple strings of photovoltaicmodules. The array is configured so that individual modules, strings orsets of strings may be selectively connected and disconnected from anconverter, allowing monitoring and diagnostic operations on a module,string, or sub array level. The array can also include a safety modewith no power output.

The disclosed innovations, in various embodiments, provide one or moreof at least the following advantages. However, not all of theseadvantages result from every one of the innovations disclosed, and thislist of advantages does not limit the various claimed inventions.

-   -   Allows diagnostics during daylight and nighttime or shaded        conditions;    -   Allows higher level diagnostic operations without additional        expensive equipment;    -   Monitoring and diagnostics may be performed without technicians;    -   Provides for string or module level monitoring and diagnostics;    -   Provides for regular monitoring during operation without        significant power loss;    -   Provides no output safe mode for maintenance or emergencies;    -   Improves array performance and efficiency;    -   Allows early detection of problems;    -   Reduces costs associated with warranty obligations of the system        manufacturer or installer;    -   Reduces maintenance costs;    -   Allows for diagnostics on module, string or sub array level        without separate equipment at each unit;    -   Allows for better visibility into the long-term life expectancy,        and in-service degradation, of a solar array, and hence allows        for lower implicit risk allowance in financing of solar array        systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments and whichare incorporated in the specification hereof by reference, wherein:

FIG. 1 schematically shows an embodiment of a PV system.

FIG. 2 schematically shows possible measurement locations for a PVsystem.

FIG. 3 is a flow chart illustrating a procedure for monitoring poweroutput of a PV array.

FIG. 4 shows an embodiment of a PV array with one string disconnected.

FIG. 5A is a flow chart illustrating a procedure for diagnosing problemsin a PV array.

FIG. 5B is a flow chart illustrating an improved procedure fordiagnosing problems in a PV array.

FIGS. 6A-6F illustrate several operating and diagnostic scenarios for aPV array.

FIGS. 7A-7C shows an embodiment of a PV array with one string connected.

FIG. 8 shows an example of an I-V curve for a properly operating PVmodule.

FIG. 9 shows examples of I-V curve abnormalities.

FIG. 10 shows an example of an I-V curve for a properly operating PVmodule at night time.

FIG. 11 shows an example of a bidirectional power converter topologywhich can be used with the disclosed inventions, and is especiallyadvantageous.

FIG. 12 shows a more comprehensive illustration of a solar power system.

FIGS. 13A-13D show example states of an embodiment of a PV stringcombiner.

FIG. 14 shows a PV system with string shorting to eliminate outputvoltage on for safety.

DETAILED DESCRIPTION OF SAMPLE EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to presently preferred embodiments(by way of example, and not of limitation). The present applicationdescribes several inventions, and none of the statements below should betaken as limiting the claims generally.

The present application describes photovoltaic power systems with anumber of new features. In the following description, it should beremembered that the various inventions can be used in other combinationsor subcombinations, or separately, and do not have to be combined asdescribed.

The disclosed inventions focus on improving system performance byenabling low-cost string-level DC monitoring and diagnostics with noreduction in peak efficiency. Improved DC monitoring and diagnosticswill enable PV system owners and operators to easily identify andcost-effectively correct non-optimal array conditions.

Several of the new functions will now be described briefly, and a moredetailed description follows later.

System and String DC Power Monitoring

AC power monitoring from the converter output is common in the industry,but DC power monitoring is less frequent due to higher implementationcost of measuring DC current. One group of disclosed inventions allows asingle DC current and voltage power monitoring system in the converter.The converter can measure combined string DC current and voltage(continuously or with sampling). This data will be stored temporarily inthe converter and downloaded to a remote internet data server.

In the preferred implementation, the DC power monitoring system willregularly disconnect individual strings and measure the change in DCcurrent and voltage. The difference in power will be the power lost fromthe disconnected string. This only requires short disconnect time(approximately 1 second per string) with the rest of the system active.As result there is no significant energy loss in this procedure. Thisfunction would be performed regularly (such as once every few hours) oras desired via remote operator control.

In a sample implementation, the string power measurements are comparedto power measurements from other strings from the same point in time andprevious measurements from the same string to determine if the power islower than predicted. This would be determined by the remote data serveror by the converter. If the difference between predicted string powerand measured string power is higher than a set point, then the systemmay be programmed to trigger a string alarm at the converter or bysending email or text message(s) to maintenance personnel.

Some industry products support string alarms, but with higher systemcosts. These systems use string level DC current meters, an expensivecomponent, on every string. These provide continuous DC currentmeasurements on every string, but practically these provide no moreuseful system information than the regularly sampled string monitoringachieved with this group of inventions.

String Diagnostics

String diagnostics would occur when a string alarm is received or duringinfrequent maintenance (perhaps monthly). String diagnostics can beperformed on all strings simultaneously, if appropriate current sensorsare provided for each string, or can be performed one string at a timeusing string disconnects with one current sensor at the centralconverter. When combined with advanced DC monitoring capabilities in theconverter, a wide variety of string measurements can be taken includingopen circuit voltage (Voc), Short Circuit Current (Isc), andcurrent-voltage (I-V) performance curve. Furthermore this could beimplemented during both daylight and night conditions.

This data will be provided to the remote data server which can be usedto determine a wide variety of string problems including shading,soiling, degrading/out of specification PV modules, and string wiringshorts/open conditions. This alerts maintenance personnel to specificproblems and corrective action requirements.

String Disconnect for String Maintenance

Disconnecting the individual string can allow maintenance to work on aspecific string while the remainder of the array is active. Replacing aPV module or looking for wiring shorts/opens is an example ofmaintenance work that can be simplified with this system.

Array Disconnect for Converter Maintenance

The system can provide DC array disconnects near the PV array while theconverter may be located some distance away. By disconnecting all PVstrings, the converter is disconnected from the DC power source. Thiscan simplify converter maintenance or replacement procedures.

Array Disconnect for Safety

Fire departments normally shut off the main AC power disconnect whenresponding to fires to ensure the electrical lines in a building arede-energized. Since PV modules generate power whenever the sun isshining, disconnecting the main AC alone does not shut off DC powerlines associated with the PV system. Many fire codes (including NEC)require DC disconnects on the roof near the PV array. The stringdisconnects in the preferred implementation of the disclosed inventionswill have a local manual switch as well as a remote converter controlledarray shutdown for safety.

PV Power Architectures with Bidirectional Power Converter

For many applications, a Photovoltaic power system will includeconnection to local AC load, power line, battery, photovoltaic array,and sometimes also to other local loads (e.g. DC, or multiple ACvoltages). As described in published PCT application WO2008/008143,which is hereby incorporated by reference, a Universal Power Conversionarchitecture is useful for such applications. This architecture providesextremely versatile conversion of voltage, current, impedance,frequency, and/or phase. The present inventors have realized that thisarchitecture opens the door to even more capabilities in PVarchitecture.

FIG. 11 shows a simple example of a current-modulation PV convertertopology, which can be used as the bidirectional converter in thevarious system implementations which will be described below. Theexample shown in this drawing is a bipolar array, where a common groundis used with two photovoltaic arrays which provide separate positive andnegative voltage outputs. However, the same converter topology can alsobe used in a single-ended configuration, as shown in the examples ofFIG. 1 and other drawings. This example shows conversion from a bipolargrounded array of photovoltaic cells to three-phase 480V AC power (e.g.at 60 Hz). The link reactance is an inductor-capacitor parallelcombination, and is operated in a current-modulation mode. In thisexample, five identical phase legs are used, each of which includes twoAC switches. Two phase legs are used for the DC lines to the PV array,and three are used to connect to the three phases of the AC outputlines. No other power semiconductors are needed.

In addition to the active components in the phase legs the converteruses a central Link Inductor (LI) with a Link Capacitor (LC) inparallel, and input and output capacitive filters. All power transferoccurs through the LI such that input and output lines are neverconnected together, which is why this topology does not need atransformer to achieve common mode isolation between the solar array andthe utility lines. The LC allows for zero voltage turn-off (ZVT), almosteliminating switches losses.

At full power, the link frequency is 7,000 Hz. The converter iscontrolled by an FPGA, which only needs line and link voltage sensing tooperate. This simple example shows a minimal configuration with only twoports (line and photovoltaic array), but practical implementations wouldtypically include more connections.

Many of the described embodiments are single-ended, where only two wires(power and ground) connect the converter to the PV array. However, thisfigure illustrates a center-grounded embodiment, where three wires(positive, ground, and negative) connect the converter to the PV array.

FIG. 12 is a more complete example, showing how a bidirectionalconverter would be connected to multiple portals in a complete PV powersystem. In this example, four portals are provided, namely “House”(local AC load), “Utility” (power line), “Backup” (battery), andphotovoltaic array. However, other portals can be added. In addition,polyphase connections can be provided instead of or in addition to thesingle-phase connections shown.

String Shorting

As photovoltaic array designs have migrated to higher DC voltage levels,safety has become an increasingly important concern. (Older designsmight use e.g. 48V DC peak daylight voltage, but newer designs commonlystack PV modules to achieve a 600V or 1000V DC voltage, for reducedloss.) The present application provides a new approach to safety: whentotal shutdown is desired, two identical (or comparable) strings can beconnected together with opposite senses, i.e. positive to negative (orground) and negative (or ground) to positive. This avoids the danger ofa high voltage appearing when illumination changes. This connection willcause a current loop, so preferably this connection is made betweenelements or subarrays which will not exceed the current rating of theswitches. Alternatively, a positive terminal of a string is shorted tothe negative terminal of the same string. An embodiment of a PV systemin safe mode with shorted strings is shown in FIG. 14.

Apply DC Voltage to Array for Monitoring/Diagnostics

One important class of teachings uses the bidirectional converter toapply a selected DC voltage back onto the PV array. This can be used ina number of ways, as described below, to provide the system withadditional capabilities.

In a system with battery backup, the converter can also be used to drawpower from the backup battery to apply a back voltage onto the array.However, this is less preferred.

Method of Diagnostics by Applying Variable DC Voltage

One important class of teachings uses the bidirectional converter toapply a variable DC voltage back onto the PV array. By ramping orstepping the applied voltage, while monitoring current, I/V curvecharacterization can be used to assess the PV array.

In a system with battery backup, the converter can also be used to drawpower from the backup battery to apply a back voltage onto the array.However, this is less preferred.

Method of Diagnostics by Applying DC Reverse Voltage

Another important class of teachings uses the bidirectional converter toapply a negative DC voltage (opposite to the voltage which is generatedin daylight) back onto the PV array at nighttime. The negative voltagepermits the bypass diodes to be tested.

In a system with battery backup, the converter can also be used to drawpower from the backup battery to apply a back voltage onto the array.

Theft Prevention

Another important class of teachings uses the bidirectional converter toapply a small DC voltage onto the PV array at nighttime, even when nodiagnostic tests are being run. This permits changes in the impedance ofthe array to be measured, to thereby detect theft of elements ormodules.

Implications for Risk Assessment and Bankability

Solar energy is an area of great interest, but the scale of solarinstallations is still relative small compared to other energy sources.As this industry rapidly grows and matures, there are some importantissues beyond the basic scientific and engineering technologies. One ofthese is the financial risk of a large solar power system, whenconsidered as a major asset.

One known issue for photovoltaic arrays is degradation over time.Sealing can degrade, leakage currents can grow worse, and thesemiconductor material itself can develop increased internal leakage(and possibly even short circuits). Connections can degrade too, leadingto open circuits or short circuits. The physics of these changes isfairly well understood, but these issues present some uncertainfinancial risk over the assets' lifetime.

For example, the initial performance of a $100 solar power kit forenthusiasts might be sufficient to justify its price, even if the unitonly lasts a few years under outdoor conditions. However, if we areconsidering a $10M capital investment, for an array which may put outmegawatts of power, more careful analysis is needed. For such aninvestment to be practical, it needs to be “bankable,” i.e. predictableenough (in its future contribution to revenue) to support financing. PVmodule vendors provide warranties for maximum degradation, but it isdifficult to measure module or array degradation accurately once thesystem is installed. To the extent that long-term degradation is notaccurately measurable in the installation, there is an additional riskfrom investors. If the degradation can be accurately and easilymeasured, the financial risk is reduced and so will the interest cost offinancing.

The disclosed inventions have a major impact on this uncertainty, sincedegradation can be monitored accurately and easily in-service. Thus aphotovoltaic power system with monitoring capabilities as describedherein will not only have a predicted initial lifetime, but thatlifetime will become less uncertain as time goes by: not only will thein-service monitoring show where individual components are, within theirlifetimes, but results from similar but older arrays will give anincreasingly accurate picture of expected lifetime and maintenance cost.

FIG. 1 schematically shows a photovoltaic (PV) system 10 for collectionof solar energy. PV system 10 generally comprises a PV array 110, astring combiner 120, and a converter 130. PV array 110 preferablycomprises a plurality of photovoltaic modules 112. A PV module istypically a generally planar device comprising a plurality of PV cells.

Several PV modules 112 are combined in series to form strings 114.String 114 preferably comprises between 8 and 15 PV modules 112.However, the number of PV modules 112 in a string 114 can vary dependingon the output voltage of each PV module 112 and the desired maximum DCoperating voltage of PV system 10. Common maximum DC operating voltagesare 600V DC and 1000V DC. Strings 114 are preferably combined inparallel at string combiner 120.

String combiner 120 preferably includes a switch 122 for each string 114in PV array 110. Switch 122 is configured to selectively connect ordisconnect string 114 from PV array 110. Each switch 122 is separablyoperable, so that one or more switch 122 can be opened (disconnectingone or more string 114 from PV array 110) while other switches 122remain closed (other strings 114 remain connected). Most preferably,switch 122 is controllable by a machine or user at a remote location.String combiner 120 also preferably comprises a fuse 124 for each string114. While switches 122 are useful for practicing certain embodiments ofdisclosed inventions, switches 122 are unnecessary for severalinnovations disclosed herein.

Under normal operating conditions, DC power from string combiner 120 isfed into PV converter 130. PV converter 130 converts the DC power to ACpower, which can be used onsite or distributed over an AC distributionsystem. PV converter 130 is preferably a bidirectional PV converter,such as the PV converter described in WO2008/008143. PV converter 130can alternatively be operated in reverse, so that PV converter 130 drawspower from an AC power distribution system, converts the AC power to DC,and delivers a DC potential to PV array 12. PV converter 130 ispreferably configured to be able to provide either a forwardpotential—that is, a DC voltage tending to induce current in the normaldirection of current flow of the PV modules, or a reverse potential,tending to induce a current in the opposite direction of normal currentthrough the PV modules or tending to retard the flow of current in thenormal operating direction.

FIG. 2 illustrates potential measurement locations for a PV system.Measurements preferably include at least voltage and current. Position Arepresents array-level measurements taken at the PV converter on the ACside. Position B represents array-level measurements taken at the PVconverter on the DC side. Position C represents string-levelmeasurements. Position D represents module-level measurements.

Certain aspects of the disclosed invention can be performed usingmeasurements taken at any of locations A, B, C, or D. Other disclosedinventions are preferably combined with measurements at particularlocations. In some embodiments, measurement of one parameter, such ascurrent, can be taken at one location while measurement of otherparameters, such as voltage, can be taken at other locations.

FIG. 3 is a flow chart showing one process for monitoring PV array 110.In step 302, real-time power output data from PV array 110, or a portionof a PV array, is measured. In step 304, real-time weather data isdetermined. In step 306, the measured real-time power output is comparedto historical power output data for similar weather conditions. If thepower output is within a predicted range, then PV array 110 continues tooperate normally. If the power output is not within the predicted range,then, in step 308, an alert condition is initiated.

Referring again to step 302, the real-time power output data measuredfor use in comparison step 306 can be data for the array, a string, or amodule. Power output data for the array is preferably measured atlocations A or B of FIG. 2. Power output data for a string is preferablymeasured at location C. Power output data for a module is preferablymeasured at location D.

Alternatively, power output data for a string can be indirectly measuredat locations A or B if the PV system is equipped with automatic switches122. In that case, one string is preferable disconnected using automaticswitch 122, while other strings continue to provide power, asillustrated in FIG. 4. Power output data from the connected strings iscompared to historical power output data from the connected stringand/or historical or recent power output data for the entire array. Ifthe power output from the connected strings is within a predicted range,the disconnected string is automatically reconnected and the next stringis tested. If the power output from the connected strings is outside ofa predicted range, an alert condition is initiated. Testing stringsusing the method described in this paragraph allows for string-levelmonitoring of a PV array without requiring expensive monitoringequipment for each string.

The response to an alert condition depends on the capabilities of the PVsystem. FIG. 5A illustrates a procedure for use in a PV system withoutautomatic diagnostic capabilities. In step 502, maintenance personnelare dispatched to the PV system location. In step 504, maintenancepersonnel perform on-site diagnostics procedures as necessary todetermine the problem. In step 506, maintenance personnel correct theproblem.

Disadvantages of the procedure shown in FIG. 5B are that maintenancepersonnel responding to the alert will have limited information aboutthe problem before arriving on site. Rather, maintenance personnel willneed to perform on-side diagnostics, which takes additional time andcost. The requirement of on-site diagnostics also means more experiencedand qualified staff must respond to the alert, even though the problemmay be relatively simple to fix. Additionally, the lack of informationincreases the likelihood that maintenance will not have the necessaryequipment to fix the problem and that a second visit will be required.These issues increase maintenance costs.

FIG. 5B illustrates a procedure for use in a PV system with automaticdiagnostic capabilities. In step 552, after an alert condition isinitiated, diagnostic procedures are performed automatically by the PVsystem or by a remote server or operator. Certain useful diagnosticsprocedures are discussed elsewhere herein. In step 554, results of thediagnostics procedures are preferably transmitted to a remote server oroperator. In step 556, the remote server or operator determines anappropriate responsive action. In step 558, a maintenance visit isscheduled. In step 562 maintenance personnel correct the problem. Theprocedure illustrated in FIG. 5B can greatly reduce maintenance costsbecause information about the problem and potential causes will beavailable before an on-site visit. This information increases thelikelihood that the right personnel and equipment are dispatched. Theadditional information can also be utilized by maintenance to betterunderstand the severity of the problem. In some case it may be morefinancially attractive to delay dispatching maintenance and combine thesite visit with normally schedule maintenance.

In the event an alert condition is initiated or detailed diagnostics isdesired for other reasons, certain inventions disclosed herein providesystems and methods for such diagnostics.

In one class of embodiments, PV converter 130 provides a DC potential tothe connected strings. PV converter 130 is preferably configured toprovide a variable positive or negative back-converted DC potential toconnected strings using power from an AC distribution system. FIGS.6A-6G illustrate a number of potential operating and diagnosticscenarios.

FIG. 6A shows a PV array under sunlight conditions with a DC voltage Vpvbeing generated by PV strings. Current is conducted in the normaldirection. Current is preferably measured, in this example, using a DCcurrent meter 602 at each string. Energy is transferred through PVconverter 130 to the AC distribution system. A graph of voltage versustime for this condition is also shown. Some random variation may bepresent, due to variation in cloud cover etc.

FIG. 6B shows a PV array under nighttime conditions with noback-converted DC voltage applied. With negligible Vpv and Vpc, nocurrent is conducted in the system, and no energy transfer occurs.

FIG. 6C shows a variable back-converted DC voltage (from Vpc=0 toVpc>Voc) applied in daylight.

FIG. 6D shows a variable back-converted DC voltage applied at nighttimein the negative and positive directions. Again Vpv is negligible.

FIG. 6E shows a small constant back-converted DC voltage (e.g. 24V)applied at nighttime in the forward direction through all strings fortheft-detection purposes. The accompanying graph shows current versustime. A change in current as shown by the dotted line potentiallyindicates tampering; for example, if one string is removed by thieves,the leakage current contribution that string will disappear. Net energytransfer is from the converter into the array, but is typically verysmall.

FIG. 6F shows a negative back-converted DC voltage applied at nighttimeto test bypass diodes.

Alternatively, to measuring current using DC current meters 602 on eachPV string, string-level diagnostics can be performed using switches 122.In this case, all switches 122 are open with the exception of the switchassociated with the string being analyzed, as illustrated in FIG. 7A.Diagnostic functions can then be performed such as the examples shown inFIG. 7B (daytime I-V curve trace of string 1) and FIG. 7C (daytime I-Vcurve trace of string 1).

FIGS. 13A, 13B, 13C and 13D illustrate another embodiment of a PV stringcombiner in several states of operation. The string combiner illustratedin FIGS. 13A-13D divides the strings (not shown) into positive andnegative subarrays. For purposes of string-differentiation powermonitoring, as discussed above in connection with FIG. 4, a string pair(consisting of one positive and one negative string) is preferablydisconnected in this embodiment, as shown in FIG. 13C. For purposes ofindividual string diagnostics, as discussed in connection with FIG. 7,only one-half of a string pair is preferably connected, as shown in FIG.13D.

For diagnostic purposes, measurements of current through the string aretaken at multiple voltages, as shown in FIGS. 6C and 6D. Preferably,current measurements are taken at a sufficient number of voltages tocreate an I-V curve trace. An I-V curve trace can provide valuableinformation about the functioning of a PV string.

FIG. 8 shows an example of an I-V curve trace of a PV string takenduring daytime with significant insolation. At V0, the PV converter isnot providing any load on the PV string and the current through PVstring is at the maximum current, also called the short-circuit current(Isc). For a normally-operating string, as the applied voltageincreases, the current through the string decreases. At lower appliedvoltages, the current is not sensitive to changes in voltage and thecurrent decreases gradually from Isc, giving a shallow slope. At highervoltages, the current decreases rapidly as the applied voltageincreases, providing a steep slope. Around the transition point is amaximum power voltage (Vmp), which is located at the point on the I-Vcurve where I*V is greatest. As the voltage increases beyond Vmp to itsmaximum (open-circuit) value Voc, the current falls rapidly to zero.

FIG. 9 is an I-V curve illustrating a number of problems that can beidentified using I-V curve analysis. Solid line 902 shows an I-V curvefor a normally-operating string. The dashed lines illustrate portions ofhypothetical IV curves corresponding to string malfunctions. Line 904has a steeper slope near V0, potentially indicating shunt losses. Shuntlosses can be caused by malfunctions such as a resistive path within acell, possibly caused by cracks or other physical damage to a cell ormodule.

Line 906 shows a notch in the I-V curve near Vmp, potentially indicatinga mismatch error in the string. A mismatch error can be caused byshading, uneven soiling, mismatched modules, or shorted bypass diodes.

Line 908 shows a curve with an unexpectedly shallow slope near V_(OC).This can indicate series losses. Series losses can be caused by problemssuch as a corroded connector.

Other features of I-V curves that can reveal potential problems arereduced I_(SC) and reduced V_(OC). Reduced I_(SC) can indicate uniformsoiling or module degradation. Reduced V_(OC) can indicate a high moduletemperature, potentially caused by poor air circulation.

FIG. 10 shows an example of an I-V curve trace (solid line) taken atnighttime, when the PV modules are producing zero or negligible power.The curve shown in FIG. 6 is preferably created by applying a variablevoltage using PV converter 130. The applied voltages preferably rangefrom a negative voltage (i.e. a potential applied in the directionopposing the normal direction of current in the array) below a V_(t)times the number bypass diodes to a positive voltage above V_(t) timesthe number of PV cells being traced. Under normal conditions, a negativecurrent will be measured at applied voltages below V_(t) times thenumber of bypass diodes. A positive current will be measured at appliedvoltages above V_(t) times the number of PV cells. Between these twopoints the current through the string will be negligible.

Examples of dark-condition curve traces that can indicate potentialproblems through a shift in these points. This can indicate an openbypass diode, which is a leading cause of arc faults in PV systems.

Alternatively to constructing an I-V curve trace, the system can takecurrent measurements at key voltages associated with the PV string. Forexample, if V_(mp) is known for a particular string, the system canmeasure current through the string at V_(mp), and the V_(mp) current canprovide useful diagnostic information.

Another use for the system is to provide highly accurate performancemeasurements. This can be useful for initial system commission andannual performance reviews. The highly accurate performance measurementscan be usefully in monitoring long term panel degradation. Currently PVsystems have relatively high interest charges, in part due to thefinancial risk in PV array degradation. By providing highly accurate PVarray degradation measurements this system may reduce financial risk andinterest costs.

According to some but not necessarily all disclosed inventiveembodiments, there is provided: A method for operating a photovoltaicarray comprising: in a first mode, providing DC current fromphotovoltaic cells to a bidirectional power converter, and operatingsaid bidirectional power converter to provide an AC power output onto anAC connection; in a second mode, operating said bidirectional powerconverter in reverse, to use electrical power from sources other thansaid photovoltaic cells to apply a DC potential to at least some ones ofsaid photovoltaic cells.

According to some but not necessarily all disclosed inventiveembodiments, there is provided: A method for operating a photovoltaicarray, comprising: in a first mode, providing DC current fromphotovoltaic cells to a bidirectional power converter, and operatingsaid bidirectional power converter to provide an AC power output onto anAC connection; in a second mode, operating said bidirectional converterin reverse, to apply a DC potential to said string of photovoltaiccells; and performing a diagnostic test on said string using the appliedDC potential.

According to some but not necessarily all disclosed inventiveembodiments, there is provided: A method for operating a photovoltaicarray comprising: in a first mode, providing DC current fromphotovoltaic cells to a bidirectional power converter, and operatingsaid bidirectional power converter to provide an AC power output onto anAC connection; in a second mode, operating said bidirectional converterin reverse, to apply a variable DC potential to said string ofphotovoltaic cells; and varying said DC potential, while measuringelectric current through said string of photovoltaic cells, to therebyobtain an I-V profile; and using said I-V profile to evaluate the stringof photovoltaic cells.

According to some but not necessarily all disclosed inventiveembodiments, there is provided: A method for operating a photovoltaicarray comprising: in a first mode, providing DC current fromphotovoltaic cells, with a first voltage polarity, to a bidirectionalpower converter, and operating said bidirectional power converter toprovide an AC power output onto an AC connection; in a second mode,operating said bidirectional converter in reverse, to apply a DCpotential to said string of photovoltaic cells, with a voltage polaritywhich is opposite to said first voltage polarity; and varying said DCpotential, while measuring electric current through said string ofphotovoltaic cells, to thereby obtain an I-V profile; and using said I-Vprofile to evaluate the string of photovoltaic cells.

According to some but not necessarily all disclosed inventiveembodiments, there is provided: A method for operating a photovoltaicarray, comprising: using a bidirectional AC-DC converter to apply a DCpotential to a string of photovoltaic cells under dark conditions;measuring a DC current through the string of photovoltaic cells;initiating an alert if the measured DC current indicates that aphotovoltaic cell in the string has been disconnected.

According to some but not necessarily all disclosed inventiveembodiments, there is provided: A photovoltaic power generation systemcomprising: a plurality of photovoltaic cells; and a bidirectional powerconverter which is operatively connected to groups of said photovoltaiccells; wherein, in a first mode of operation, said bidirectional powerconverter draws current from at least some ones of said groups ofphotovoltaic cells, and provides an AC power output onto an ACconnection; and wherein, in a second mode of operation, saidbidirectional power converter draws power from one or more sources otherthan said photovoltaic cells, and accordingly applies a DC potential toat least some ones of said photovoltaic cells.

According to some but not necessarily all disclosed inventiveembodiments, there is provided: A photovoltaic power generation systemcomprising: a plurality of photovoltaic cells; and a bidirectional powerconverter which is operatively connected to groups of said photovoltaiccells; wherein, in a first mode of operation, said bidirectional powerconverter draws current from at least some ones of said groups ofphotovoltaic cells, and provides an AC power output onto an ACconnection; and wherein, in a second mode of operation, saidbidirectional power converter draws power from one or more sources otherthan said photovoltaic cells, and accordingly applies a variable DCpotential to at least some ones of said photovoltaic cells; and whereinone or more diagnostic tests are performed during sais second mode.

According to some but not necessarily all disclosed inventiveembodiments, there is provided: photovoltaic power systems which performany of the innovative methods described above.

According to some but not necessarily all disclosed inventiveembodiments, there is provided: Devices, systems and methods foroperating, monitoring and diagnosing photovoltaic arrays used for solarenergy collection. The system preferably includes capabilities formonitoring or diagnosing an array, under some circumstances, by using abidirectional power converter not only to convert the DC output of thearray to output power under some conditions, but also, for diagnosticoperations, applying a back-converted DC voltage to the array.

Modifications and Variations

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given. It is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

For example, while the preferred embodiment uses silicon photovoltaicdiodes, other semiconductor materials can be used, e.g. Si_(0.9)Ge_(0.1)or other SiGe alloys, or III-V or other semiconductor materials.Amorphous, polycrystalline, or single-crystal materials can be used.

A variety of structures have been proposed to gather solar energy,including structures using concentration, wavelength conversion, andmulticolor (multi-bandgap) structures, and the disclosed inventions areapplicable to all of these.

As noted above, the Universal Power Converter (UPC) topologies ofWO2008/008143 are particularly advantageous, since they provide greatflexibility in conversion of power from any portal to any other portal,in a multi-portal converter. However, this specific family of topologiesis not required for most of the disclosed inventions. Many convertertopologies can provide bidirectional (or multidirectional) transfer ofpower, even if the full flexibility of the UPC topologies is notpresent, and some of these bidirectional-power-transfer topologies canbe used for some of the claimed inventions. (For example, a simpleisolated buck-boost topology can be operated to provide bidirectionalpower transfer, e.g. as in starter/generator systems for aircraftengines.) Some of the disclosed inventions are useful even if abidirectional converter is not present. Of course, a great variety ofvariations are possible within the basic UPC topology too.

The particular requirements of different applications can also beaccommodated by appropriate customization of different portals of asingle converter. For example, in some applications it might be usefulto have a 12V DC output for standard automotive accessories, as well asa 48V DC output for connection to a battery bank, a 120V 60 Hz outputfor standard consumer or office appliances, and/or a 5V DC output fordeicing through exposed connections.

None of the description in the present application should be read asimplying that any particular element, step, or function is an essentialelement which must be included in the claim scope: THE SCOPE OF PATENTEDSUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED CLAIMS. Moreover, none ofthese claims are intended to invoke paragraph six of 35 USC section 112unless the exact words “means for” are followed by a participle.

Additional general background, which helps to show variations andimplementations, as well as some features which can be synergisticallywith the inventions claimed below, may be found in the followingpublications and applications: Balakrishnan et al., Soft switched aclink buck boost converter, in Applied Power Electronics Conference andExposition 2008, pp. 1334-1339.; Balakrishnan et al., Soft switchedac-link wind power converter, in IEEE International Conference onSustainable Energy Technologies 2008, pp. 318-321; Toliyat et al., Softswitched ac-link AC/AC and AC/DC buck-boost converter, in PowerElectronics Specialists Conference 2008 pp. 4168-4176; and U.S.application Ser. No. 13/205,243 (pending), which is a continuation ofSer. No. 12/479,207 (pending, and published as US 2010-0067272), whichis a continuation of Ser. No. 11/759,006 (now issued as U.S. Pat. No.7,599,196), which is a nonprovisional extension of 60/811,191 filed Jun.6, 2006 (and now expired); WO 2008/008143; and U.S. Pat. No. 7,778,045).All of these applications have at least some common ownership,copendency, and inventorship with the present application, and all ofthem are hereby incorporated by reference.

The claims as filed are intended to be as comprehensive as possible, andNO subject matter is intentionally relinquished, dedicated, orabandoned.

What is claimed is:
 1. A method for operating a photovoltaic arraycomprising: in a first mode, providing DC current from photovoltaiccells to a bidirectional power converter, and operating saidbidirectional power converter to provide an AC power output onto an ACconnection; in a second mode, operating said bidirectional powerconverter in reverse, to use electrical power from sources other thansaid photovoltaic cells to apply a DC potential to at least some ones ofsaid photovoltaic cells; wherein, in said second mode, said DC potentialis varied to provide an IV curve for ones of said cells.
 2. A method foroperating a photovoltaic array, comprising: in a first mode, providingDC current from photovoltaic cells to a bidirectional power converter,and operating said bidirectional power converter to provide an AC poweroutput onto an AC connection; in a second mode, operating saidbidirectional converter in reverse, to apply a DC potential to thestring of photovoltaic cells; and performing a diagnostic test on saidstring using the applied DC potential; wherein, in said second mode,said DC potential is varied to provide an IV curve for ones of saidcells.
 3. A method for operating a photovoltaic array comprising: in afirst mode, providing DC current from photovoltaic cells to abidirectional power converter, and operating said bidirectional powerconverter to provide an AC power output onto an AC connection; in asecond mode, operating said bidirectional converter in reverse, to applya variable DC potential to the string of photovoltaic cells; and varyingsaid DC potential, while measuring electric current through said stringof photovoltaic cells, to thereby obtain an I-V profile; and using saidI-V profile to evaluate the string of photovoltaic cells.
 4. The methodof claim 3, wherein, in said second mode, said bidirectional converteris powered from said AC connection.
 5. The method of claim 3, whereinsaid second mode is initiated automatically, from time to time, duringnormal operation.
 6. The method of claim 3, wherein said second mode isinitiated automatically, from time to time, during dark conditions.